In Siberia, the permafrost thaws, releasing vast frozen
stores of greenhouse gases that send temperatures even higher.
In the tropics, the Amazon rainforest starts to die off because
of a warmer, drier climate.

Such scenarios may read like the script of a Hollywood
disaster movie but many scientists say there are real risks of
"tipping points" -- sudden, catastrophic changes triggered by
human activities blamed for warming the planet.

"Even small risks in the climate need to be considered,
just as we try to avert accidents at nuclear power plants,"
said Stefan Rahmstorf, a professor at the Potsdam Institute for
Climate Impact Research and an expert in ocean currents.

"I don't think this is scaremongering. We don't really
understand the system," he said of risks that the warm Gulf
Stream current in the North Atlantic might shut down in one
possible "tipping point" scenario.

Melting ice in Greenland could send a sudden flow of cool
water into the North Atlantic, disrupting the giant current
that pulls warm water northwards to create the Gulf Stream.

This might shut down the warm current and could also make
parts of Europe and North America sharply colder, despite an
overall warming of the climate.

Scenarios like this, and the uncertainty surrounding them,
will provide a dramatic backdrop to a United Nations climate
change meeting in Montreal, Canada, from November 28-December
9.

Around 190 countries will debate how to expand a U.N.-led
fight against global warming to include developing nations such
as China and India and skeptic countries, led by the United
States and Australia.

Under the U.N.'s Kyoto Protocol, about 40 rich nations have
agreed to curb emissions of heat-trapping gases released by
burning fossil fuels in power plants, factories and cars by 5.2
percent below 1990 levels by 2012.

What will happen after 2012 is unclear.

HISTORY LESSONS

Many environmentalists say the risk of "tipping points"
makes it ever more urgent to curb climate change, already
widely predicted to cause more storms and floods and even drive
some species of animals and plants to extinction.

But there are those who disagree.

"Environmentalists talk about 'tipping points' because they
are frustrated," said Fred Singer, head of the U.S. Science and
Environmental Policy Project. He believes humans can adapt to
any warming caused by a buildup of greenhouse gases.

"All the climate models that I've seen show only a gradual
warming as the level of greenhouse gases increases," he said.

Even so, records of the ancient climate found in ice caps
and ocean sediments show there have been staggeringly big
shifts in the past. "Past climate change is ringing alarm
bells," Rahmstorf said, referring to the climate's fragility.

During the last Ice Age, temperatures in the North Atlantic
region once soared by 12 Celsius (22 Fahrenheit) in just 10
years, perhaps because of swings in ocean currents linked to
small shifts in the sun.

Such drastic changes have stopped since the end of the Ice
Age about 10,000 years ago, maybe because ocean currents are
more stable outside Ice Ages, according to a study co-authored
by Rahmstorf and published in the Nature journal this month.

There are more obvious examples of "tipping points" in
nature, like the collapse of cod stocks off Newfoundland,
Canada, in the early 1990s from overfishing.

And, at some point in the 17th century, hunting of the
flightless dodo in Mauritius doomed the birds to extinction.

Concerns about "tipping points" today focus on the Arctic.

Experts say Greenland's 3,000 meter (9,800 ft) thick ice
sheet, which has been melting at ever higher altitudes in
summers in recent years, may be vulnerable to a runaway thaw.

If the Greenland sheet melted entirely over the next few
centuries, world sea levels would rise by about 7 meters (23
ft). Antarctica's far bigger ice cap is likely to be more
resilient as the giant continent acts as a deep freeze.

A melting of the Arctic "may happen very abruptly. It's one
of the big unknowns and would be irreversible," said Paal
Prestrud, head of the Center for International Climate and
Environmental Research in Oslo.

RISKS BY 2050

"The concern is that there are tipping points out there
that could be passed before we're halfway through the century,"
said Tim Lenton, an earth systems modeler at Britain's
University of East Anglia.

Risks of abrupt change seem greatest in the Arctic, where
warming uncovers dark ground or sea that soaks up more heat
than reflective snow and ice, accelerating the melt, he said.

Other reports have pointed to risks that Siberia's
permafrost could start to thaw, releasing heat-trapping
methane, or that the Amazon rainforest could shrink.

There could be some benefits from change: Lenton said some
models suggested a shift of monsoon rains in West Africa toward
the Sahara desert, possibly making that region more fertile.

Assessing risks of "tipping points" is almost impossible.

Rahmstorf said he recently polled 12 experts on the chances
of a collapse of the Gulf Stream: four said risks were above 50
percent if world temperatures rose by 5C (9F) by 2100.

"That was unexpected for me, I reckon the risks are lower,"
he said of the so far unpublished survey. A rise of 5C is at
the top of a range forecast for global warming by 2100 by the
scientific panel that advises the United Nations.